1. Field of the Invention
The present invention relates to a heat shield for an aircraft, and in particular to a heat shield for an aircraft wheel and brake assembly.
2. Background of the Invention
Aircraft brakes operate on the basis of converting mechanical energy into thermal energy to stop an aircraft. Such braking operations generally result in the generation of significant heat within a brake disk stack, and the transfer of a significant portion of this heat to other components of the wheel and brake assembly.
FIG. 1 illustrates a conventional landing gear 1 that includes a strut 2 and a bogie beam 3. An axle 4 is provided on ends of the bogie beam 3. The axles 4 are each adapted to receive a wheel assembly 5 and a brake assembly 6. The brake assembly 6 includes a brake stack (not shown in FIG. 1) that has a plurality of alternating interleaved stator and rotor disks, the stator disks being affixed to the axle and the rotor disks being affixed to the wheel assembly. Brake actuation devices, such as pistons and return mechanisms, achieve a forceful inter-engagement of the rotor and stator disks in order to slow and stop the rotation of the wheel assembly. As such, the brake stack generates a considerable amount of heat energy that must be dissipated.
The successful development of carbon discs or carbon composite discs for aircraft brakes has significantly elevated the operating temperature of the wheel and brake assembly in contrast to steel discs. For example, modern aircraft brakes can attain a brake stack temperature of 1000-2000° F. after landing and taxiing into a gate. Thus, it is important to limit the heat transfer to the wheel assembly and the axle, in particular because excessive temperatures can damage the wheel, which loses strength at around 400° F.
Because of the high temperatures induced in the brake stack, heat shields are often used to thermally isolate the brake stack from the wheel assembly, bearings, pistons, axle, and other adjacent structures. In other words, these heat shields are used to prevent convection and radiation of heat energy to the wheel assembly that is generated by the brake stack. One skilled in the art recognizes that convection is defined as the transfer of heat energy through a medium by the circulation of currents from one region to another and that radiation is defined as the emission and propagation of heat energy in the form of rays or waves, which requires no medium for such transfer. In other words in order to radiate energy from one surface to another they must be able to see on another, e.g., there must be a direct line of sight.
FIG. 2 illustrates a conventional heat shield 7 that is provided between a brake stack 8 and the wheel assembly 5 in order to thermally isolate the wheel assembly 5 from the thermal energy (heat) that is generated by the brake stack 8 during a braking operation. These conventional heat shields 7 have a variety of configurations, however, they are commonly configured from two layers of sheet metal that have an insulating filler sandwiched between them.
These heat shields 7, however, produce an undesirable side effect in that the heat shields 7 retard heat loss from the brake stack 8. Thus, the brake stack 8 cools more slowly, thereby remaining at elevated temperatures. Such a complete blocking of the heat energy transfer is not desirable, as this leads to heat concentration in the brake stack 8, which can lead to premature failure under heavy-duty brake applications, for example, as in an abortive take-off. Furthermore, because the brake stack 8 remains at an elevated temperature, routine brake maintenance is hampered and thus departures of the aircraft from an airport gate are delayed.
U.S. Pat. No. 5,107,968 discloses a honeycomb open cell structure that limits the radiant and convection heat energy transfer from the aircraft brakes. These honeycomb heat shield structures are formed by stacking a plurality of stamped metal sheets onto one another, with each metal layer having their stamped sections being placed onto the next metal layer in an inverted fashion, thereby forming the open cell structure. By this configuration, a partial amount of radiant heat energy is transferred from the aircraft brakes to an aircraft wheel because there is a direct line of sight between the aircraft wheel and the aircraft brakes through the open cell structure of the honeycomb shield. This honeycombed shield structure, however, is thick, cumbersome, fragile, and expensive to manufacture.
Thus, there remains a need for heat shields that are configured to increase the cooling rate of aircraft brakes, are economical, durable, and are adapted to fit within the limited space provided between the brake assembly and the wheel assembly.